Conventionally, for example, a motor which has first and second rotors concentrically provided around the axis of rotation of the motor and which controls the peripheral relative positions, i.e., phase difference of the first and second rotors according to the rotational speed of the motor or according to the speed of a rotating magnetic field generated in a stator is known (for example, refer to Patent Document 1). In this motor, for example, in a case where the phase difference of the first and second rotors is controlled according to the rotational speed of the motor, the peripheral relative positions of the first and second rotors are changed via a member which is displaced along a radial direction by the action of the centrifugal force. Further, for example, in a case where the phase difference of the first and second rotors is controlled according to the speed of a rotating magnetic field generated in the stator, the speed of a rotating magnetic field is changed by applying a control current to stator windings in a state where the rotational speed of each of the rotors is maintained by inertia, thereby changing the peripheral relative positions of the first and second rotors.
Meanwhile, in the motor related to an example of the above conventional technique, for example, in a case where the phase difference of the first and second rotors is controlled according to the rotational speed of the motor, there occurs a problem that the phase difference of the first and second rotors can be controlled only in the operating state of the motor, i.e., only in a state where the centrifugal force according to the rotational speed of the motor acts, and the phase difference cannot be controlled with proper timing including a stopped state of the motor. Further, in a state where the vibration from the outside is apt to act on the motor like a case where the motor is mounted on a vehicle as a driving source, there occurs a problem that it is difficult to properly control the phase difference of the first and second rotors only by the action of a centrifugal force. Furthermore, in this case, the phase difference is controlled irrespective of fluctuation in a power supply voltage in a power source for the motor. Therefore, there is a possibility that, for example, the magnitude relation between the power supply voltage and the reverse electromotive voltage of the motor may be reversed. Further, for example, in a case where the phase difference of the first and second rotors is controlled according to the speed of the rotating magnetic field generated in the stator, the rotating magnetic field speed is changed. Therefore, there occurs a problem that the control processing of the motor may be complicated.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-204541